1. Field of the Invention
The present invention relates to low impedance, low-noise power buses for power electronics applications. Specifically, the present invention relates to power buses for high power switching inverters (DC-to-AC) and converters (DC-to-DC or AC-to-DC), and in particular, to power bus topologies and interconnections for DC-to-AC high power inverters that have a plurality of switching devices and produce three phase AC power.
2. Description of the Related Art
Power electronics is a multi-disciplinary field that deals with electrical power equipment, electronic devices for converting and controlling the electrical power, and circuits for controlling the electronic devices to provide the desired output power. Electrical power equipment may include static and rotating power equipment for the generation, transmission, and distribution of electric power. Electronic devices may include solid-state semiconductor devices. Control issues include the steady-state and dynamical characteristics of closed-loop systems. Power electronics circuits may include circuits that produce DC, AC, or other waveforms to be used as power sources for other electrical components. In general, power electronics circuits process power for delivery to other circuits; by comparison, circuits for signal processing process a signal to provide a desired output. One example of a circuit for signal processing is an integrated circuit.
The field of power electronics includes applications of solid-state electronics for control and conversion of electric power. A circuit that can convert electrical energy from DC (direct current) to AC (alternating current) is termed an inverter. Such an inverter has many uses, including wind turbines, adjustable speed motor drives, uninterruptible power supplies, and variable frequency AC power supplies. For wind turbines that supply electrical energy to a power grid, an efficient DC-to-AC inverter is expected to be very useful in the next generation of wind power devices, particularly those designed for variable speed operation. Also, an efficient AC-to-DC converter will be very useful; many DC-to-AC inverters can be operated in reverse as an AC-to-DC converter. Basic issues in wind turbine generation are conversion efficiency and reliability. For wind turbines, efficiency of the conversion process directly affects the economics and profitability of power generation; a lack of efficiency can undermine the profitability of a wind turbine project. At a very low efficiency, a wind turbine project becomes unprofitable to install and operate. Furthermore, reliability is an important concern for wind turbines deployed in the field; repair costs are expensive and down time can represent a major loss of income while the turbine awaits repair.
Generally speaking, a conventional DC-to-AC inverter employs a number of switching cells, each of which may include one or more semiconductor devices, connected between a DC power supply and an AC power output. The switching cells are switched on and off to create a desired waveform. For example, the switching cells may be selectively turned on and off many times to produce the desired output AC frequency, such as 60 Hz. An inherent problem with switching the cells, particularly for high power uses, is the appearance of large voltage and current transients. The voltage and current transients occur during the semiconductor switching process, and result from stray parasitic inductances, which are discussed in more detail later in the background. Generally, these large transients can cause noise and can interfere with the functioning of low power components nearby, as well as reduce the life of electrical components subject to the transients. In addition, the transients reduce inverter efficiency and require the design to include more expensive components that can withstand the high voltage peaks of the transients. As will be explained later in more detail, additional electrical circuits, such as snubber networks, have been used to reduce the transient's effects.
A conventional DC-to-AC inverter circuit will be described. Referring to FIG. 1, a conventional DC-to-AC power inverter system 10 generally includes a DC power source 12 and a switching matrix 14. The DC power source 12 may have series-connected DC power sources 16,18 providing positive +DC and negative -DC voltages with respect to an isolated ground 20. The switching matrix 14 includes a plurality of current switching cells, each including a semiconductor device. Many different types of semiconductor devices may be used; these include bipolar junction transistors (BJTs), silicon controlled rectifiers (SCRs), gate turn off thyristors (GTOs), insulated gate bipolar transistors (IGBTs), or metal oxide silicon field effect transistors (MOSFETs). The switching cells are connected as shown across the +DC and -DC power lines, and are selectively switched on and off to produce three-phase AC power. As shown in FIG. 1, a total of six switches are used, and therefore a total of six trigger signals provided by a controller (not shown) in a conventional pattern to convert DC power into three-phase, AC power. For purposes of explanation, the six switches can be divided into three pairs; each pair producing one of the three phases. Also for purposes of explanation, DC power is input to the switches, and AC power is output from them. However, the same circuit could easily operate in reverse; i.e., the AC power could be the input, and the DC power could be the output. The switches are typically operated to provide AC (alternating current) at a frequency of 60 Hz, however, the switches may also be operated to provide AC at other frequencies, or even to provide waveforms other than AC.
The first phase, i.e. phase A, is generated using the pair of current switching cells CSDA,CSDA. The switch CSDA is connected between the +DC bus and the phase A line, and the switch CSDA is connected between the phase A line and the -DC bus. As is known in the art, by providing appropriate trigger signals TA/TA to the current switching cells CSDA,CSDA, an AC voltage is generated at the phase A line. The two remaining phases (i.e., phases B and C) are similarly produced with their own current switching cell pairs CSDB/CSDB,CSDC/CSDC and trigger signals TB/TB,TC/TC.
This type of DC-to-AC power inverter is used in numerous applications, including high power circuits where relatively high DC voltages (e.g. 650 volts) must be converted to AC voltages. Such high power applications require the use of high power current switching cells and high current capacity power lines for the +DC and -DC buses and AC power lines.
A physical embodiment 30 of the switch matrix is shown in FIG. 2. As shown, each current switching cell includes a pair of transistor modules; for example, the current switching cell CSDA includes a transistor module Q1A and a transistor module Q3A. The current switching cells are interconnected according to the schematic of FIG. 1 via DC bus bars 34,34 for the +DC and -DC lines, and AC bars 36,38,40 for the AC phase A, phase B, and phase C connections. The bus bars 32,34,36,38,40 must have sufficient conductivity and physical size to accommodate the current levels used.
The transistor modules are commercially available, and designed for this simple bus bar interconnection approach. The appeal of this approach is both economical and practical; a variety of circuit topologies, including the inverter of FIG. 1, can be fabricated from a few simple metal bars and available modular components. This bus bar interconnection approach has been successfully applied with BJTs, SCRs and GTOs, all of which have relatively slow switching speeds. Problems arise, however, when this simple, economical approach is attempted with higher switching speed devices such as MOSFETs and IGBTs, and particularly when larger currents are being switched. High rates of change in current, on the order of 300 A/.mu.sec-2000 A/.mu.sec, appear in the faster devices. As will be explained later in more detail, the high rate of change of current (i.e. high di/dt) interacts with the relatively high characteristic inductance (L) of the bus bar to cause an excessively high voltage transient (V=Ldi/dt). High voltage transients stress the blocking voltage capability of the current switching cells. In order to withstand these transient voltages, the current switching cells must include higher voltage transistors, which are more costly. Furthermore, the voltage transients reduce reliability and decrease overall inverter efficiency. Another problem with the bus bar interconnection approach shown in FIG. 2, which will be described in more detail later, is that the bus bar structure serves as a source of electromagnetic emissions (noise) which can interfere with local low level trigger and control electronics.
The high characteristic impedance of the bus bar topology in FIG. 2 introduces significant parasitic inductances both in the DC bus circuit, and in the three phase branches. FIG. 3 is a schematic diagram of the DC-to-AC inverter, illustrating these parasitic inductances. When any current switching cell turns off, a voltage is generated across the inductances carrying that current that is proportional to the value of inductance and proportional to the rate of change of current in the device (V=Ldi/dt). If this transient is not controlled, a device failure is likely. As is known in the art, "snubber" networks may be used to control the switching transients.
A snubber network typically includes a number of passive elements interconnected within a pair of switching cells in the matrix 14. Reference is made to FIG. 4, which shows the pair of switching cells having an upper switching cell CSDU connected to the +DC bus and a lower switching cell CSDL connected to the -DC bus. A phase line is provided between the switches CSDU,CSDL. A discharge restraint snubber network SN includes diodes D1,D2, two capacitors C1,C2 and two resistors R1,R2 connected as shown between the +DC bus, the -DC bus, and the phase output. A series connected capacitor C1 and diode D1 are connected in parallel across the upper current switching cell CSDU. A similar series-connected capacitor C2 and diode D2 pair are connected in parallel across the lower current switching cell CSDL. A resistor R1 is connected to the anode of the diode D1, connecting the capacitor C1 to the -DC bus. Another resistor, R2 is connected to the cathode of the second diode D2, connecting the capacitor C2 to the +DC bus.
The snubber network SN operates as follows. After one of the current switching cells is turned off, the energy stored in the bus parasitic inductance is transferred to the respective snubber capacitor, which results in a controlled build up of capacitor voltage. The capacitor's charge is dissipated by the respective snubber resistor, causing an eventual reduction to the bus voltage across the capacitor. The dissipated energy is given up in the form of heat, evidencing a reduction in inverter efficiency. This reduced efficiency is traceable back to the parasitic inductance and characteristic impedance of the bus.
Additional problems with snubber networks include the cost of the additional components and the space requirements imposed by the circuitry. For the snubber network to effectively reduce voltage transients, the network should be located as physically close to the current switching cell as possible to minimize parasitic inductances between the snubber network and the current switching cell under protection.
Another problem with the bus bar structure shown in FIG. 2 stems from fact that the bus bar also serves as a radiating source of electromagnetic emission, a property that is traceable to the relatively high characteristic impedance of the bar. In operation, the current switching cells turn on and turn off frequently, and they generate a high rate of change of current (high di/dt). The high rate of change of current causes magnetic fields to radiate from the bus bar. The magnetic fields can interfere with sensitive low level electronics located in the vicinity of the bus bar structure. This problem is particularly acute when MOSFETs and IGBTs are used, because these devices quickly switch current at high rates of change, and require sensitive trigger circuits to be located as physically close as possible. The interfering magnetic fields can lead to mis-triggering of switching cells and unpredictable inverter operation.
As will be recognized from the foregoing, it would be desirable to provide a means for interconnecting large current switching cells in a manner that minimizes the resulting characteristic impedances. A low impedance, high power bus will provide reduced voltage transients, lower cost, higher efficiency and electromagnetic compatibility with low level switching electronics.